1. Field of the Invention
The present invention relates to a method of producing a porous metal and a metal product using the same such as a catalyst carrier and a filter element.
2. Discussion of the Related Art
Conventionally, in order to make NO.sub.x or the like contained in exhaust gas from engines nontoxic, use is made of catalyst converters using platinum as a catalyst. As the carrier for supporting the catalyst, ceramics and metal plates that are molded into a honeycomb structure are known.
Ceramic catalyst carriers are obtained by molding a raw material into a honeycomb structure and baking it. Also, catalyst carriers made of metal plates are constructed by placing a corrugated metal band and a flat metal band one over the other and rolling them as disclosed, for example, in Japanese Unexamined Patent Publication No. 71547/1987.
Also, the tendency of desire for cleaning the environment has recently increased, and it is even desired to eliminate black smoke released from engines, particularly from diesel engines that use light oil as fuel. The black smoke is composed of carbonized fine particles. Therefore, it is possible to eliminate them by a filter element having openings smaller than the particle diameter.
However, since the above catalyst converter does not have any filtering function, it cannot be used to treat black smoke simultaneously with the treatment of gases such as NO.sub.x. If the catalyst converter is made to have a filtering function, clogging will take place immediately, and as a result not only the function of the catalyst converter itself lowers but also the exhaust system of the engine is choked thereby causing the ability of the engine itself to lower.
As filter elements having openings smaller than the above carbonized fine particles, those made of a porous sintered metal or ceramic are known. The known filter elements are used in food manufacturing processes and other manufacturing processes.
However, the above ceramic catalyst carrier is attended with drawbacks that it is weak in impact and is liable to be broken.
The above metal plate catalyst carrier has a drawback that the cost is high because adjacent parts of the two metal plates are needed to be welded and a large-sized press apparatus is needed for corrugating a metal band. Further, the Fe-Al metal used in the catalyst carrier involves a manufacturing problem that it is difficult to roll.
It is impossible to arrange the above known filter element in a limited space such as the exhaust system of an engine with the filtering area made large. This is because the known filter element only has a flat filtering surface or at most only has a single cylindrical shape.
Further, a filter element used in the exhaust system of an engine must be one that can be easily regenerated. If the filter element cannot be regenerated and is replaced at every time of its clogging, not only the cost becomes so high that it cannot be adopted but also a problem that replacing work is needed arises.